Electrostatic loud-speaker



March 14, 1933. A. A. THOMAS ELECTROSTATIG LOUD SPEAKER Filed April 30,1929' AHA INVENTOR Patented Mar. 14, 1933 PATENT! OFFICE .ADOLPH A.THOMAS, F NEW YORK, N. Y.

ELEc'rRos'rATIc V.appliamon mea April so,

My invention is for a novel construction of electric loudspeakerapparatus particularly adapted for vtalking movies to improve thereproduction of sound. As far as I am aware,

the only type of loudspeaker 'used at the present time in theatersexhibiting talking ymovies is the old-fashioned horn. Usually more thanone horn is used, these horns being arranged in proximity to the screen.Since the horns must be concealed from the audience, they have to becovered in some way and that tends to muiiie the sound. But aside fromthat, owing to the large volume of sound required in theaters, theloudspeaker horns must be of great size, and it is a matter ofexperience that big horns with necessarily long sound passages give arumbling unnatural reproduction, often lacking in clearness. In sometalking movies it is sometimes impossi- '20 ble to distinguish betweenmens voices and womens voices, and between the various instruments of anorchestra. Unless the speech and music (particularly the dialogue)accompanying a motion picture are reproduced 2- in a clear and naturalmanner, an audience finds no enjoyment in thestory of the picture. Thereare, indeed, many people who object to talking movies on that accountand avoid them.

Itis the purpose of this invention to improve the reproduction ofsoundin talking movies by the use of electrostatic loudspeakers arrangedclose to the screen and directly Open to the auditorium. Where thescreen 5 is on a curtain adapted to be raised and lowered, as in largetheaters having a stage, I

support one or more condenser speakers on the curtain itself inpermanent relation to the screen. This arrangement has the practicaladvantage that the loudspeakers do not have to be handled at all, forthey move out of the way when the screen is raised. In one embodiment ofmy invention, I use several condenser speakers surrounding the screenyand constructed to accentuate different bands of frequencies.l Thisenables me to reproduce the entire musical scale with substantiallyuniform efficiency and fidelity. Owing .to their flat construction,these loudspeakers LOUD-SPEAKER,

1929. serial No. 359,235.

on an exaggerated scale to show the mountl in of certain parts inthestructure of Fig. 2;

ig. 5 is-a partial front view of the loudspeaker constructionillustrated in Figs. 2-4; and

Fig. 6 shows a side section of a modified 79|.

form of condenser speaker in which the vibratory electrode is behindaperforated section of the curtain.

A theater curtain 10 carries a motion picture screen 12 of any approvedconstruction. 75 These screens are usually an integral part of thecurtain itself, but sometimes they consist of special material attachedto the curtain., The particular construction of screen 12 and itsmounting on c'urtain 10 are imma- 80 terial in reference to the scope ofmy invention. The curtain 10 carries loudspeaker mechanism of theelectrostatic type adjacent to screen 12. In the articular embodimentillustrated in Fig. 1, this loudspeaker mech- S5 anism comprises aplurality of condenser I speakers, marked A, B and C, surrounding thescreen 12. These loudspeaker units have vibratory electrodes ofdifferent size to accentuate correspondingly dierent bands offrequencies. For example, the three upper speaker units A havediaphragms for accentuating high notes, the two lower speakers B areadapted to emphasize the middle register, and the two side speakers C,which are the largest, are intended to bring out the low notes. In thisway, all the frequencies are brought out with substantially uniformeiiciency and fidelity over the entire musical scale wthn practicallimits.

In the broad aspect of my invention, the condenser speakers may be ofany approved construction, and by way of example I have illustrated asimple form of speaker which is so flat that its presence on the curtainwould hardly be noticed by the audience. Referring to Figs. 2-5, thereis a rigid condenser plate 13 and a. vibratory electrode 14, which is atensioned sheet adapted to operate as a loudspeaking diaphragm. Theplate 13 is a rectangular piece of sheet metal, preferably aluminum,which is light and stiff, altho'ugh other metals may be used, such asbrass, iron,

'- and the like. In the present instance, the

diaphragm 14 is mounted directly on the fixed plate 13 by a frame 15,which may be of insulating material or a light strip of metal. The rigidplate 13 is provided with ribs 16 which serve to tension the diaphragm14 and at the same time hold it spaced from the fixed electrode 13. TheYribs 16 may be arranged in the outline of a rect-angle to engage thetensioned diaphragm sheet all around, or they may be provided at onlytwo opposite edges of plate 13, in which case the diaphragm will beentirely free at two edges.

The diaphragm 14 may be a thin sheet of flexible metal, such asduralumin. steel, nickel, invar (an alloy of steel and nickel), brass,and so on; or it may consist of an insulating sheet, like fabric,rubber, Celluloid, etc., covered on the outside with a coating or othersuitable metallic surface. If the diaphragm sheet 14 is of met-al, itwill have to be insulated from the metal plate 13 in any practical way.The space 17 between the condenser elements 13 and 14 may be filled withelastic dielectric material adapted to permit free and full vibration ofthe diaphragm. Screws 18 or other fastening devices hold the edge ofdiaphragm 14 clamped between the frame 15 1 and the outer Hanges 19 ofplate 13. A second frame 20, preferably of insulating material, ismounted on the back of curtain 10 and holds the condenser speakerrigidly attached to the front side of the curtain. Screws or likevmembers 21 hold the frame 20 and condenser plate 13 rigidly connectedon opposite sides of curtain 10. Two binding posts 22 and 23 projectfrom the rear frame 20 for conveniently connecting the condenserelements 13 and 14 in a circuit of variable potential. The binding post-22 is electrically connected to the fixed plate 13, while the bindingpost 23 is in metallic contact with diaphragm 14 by means of a screw orbolt 24. It is here assumed that the clamping frame 15 is of metal andin electrical contact with diaphragm 14 over a wide area.

In the modification of Fig. 6, the condenser speaker comprises a fixedplate 25 and a vibratory electrode 26, wh-ich constitutes aloudspeakingdiaphragm. What has been said about the diaphragm 14 ofFigs. 2-5 may be considered applicable to diaphragm 26 without the needof repetition. The edge of diaphragm 26 is clamped between a pair ofinsulating frames 27 and 28, which are secured to the rigid plate 25 inany practical way, as by screws, bolts or rivets (not shown). In thisconstruction the condenser unit is mounted on the back side of curtain10 and is held in place by an insulating frame 29, which is arranged onthe front side of the curtain and connected to the condenser unit byscrews 30 or otherwise. The curtain 10 is perforated at 31 to permitfree passage of sound into the open air.

It may be assumed that the specific constructions of condenser speakerabove described are applicable toI all the units A, B and C of Fig. 1,although it is not essential in actual practice that the differentspeakers be constructed exactly alike. If the dia.- phragms are arrangedon the front side of the curtain and directly exposed to view, they maybe painted or covered with a material that harmonizes with the design ofthe supporting curtain 10, so that the presence of the speakers will notbe noticed by the audience. Leastwise, the speakers will be regarded assome kind of decoration around the screen 12. Although I have shownseven loudspeaker units in Fig. 1, my invention is not limited to anyparticular number of speakers nor to any precise arrangement thereof inrelation to the screen. It is desirable to mount the speakers as closelyto the screen as possible, so as to create the illusion that the soundis coming from the characters in the picture.

The curtain 10 also performs the useful function of an acoustic baiiiewhich prevents the sound generated by the speaker units from passingback of the curtain, so that all sound waves are projected forwardlyltoward the audience. Since the diaphragms 14 and 26 are directly open tothe auditorium, the reproduction is clear and natural. It is assumed, ofcourse, that the loudspeakers are properly connected in circuit. I havenotconsidered it necessary to show or describe any circuit connections,because they will be understood by experts in this art. In largetheaters having a stage on which the settings are frequently changed,the curtain 10 will be mounted for raising and lowering, carrying withit the screen 12 and the loudspeakers A, B and C. The latter aretherefore always in proper relation to the screen without attention bythe stage hands. The curtain 10 may also be a fixed support for thescreen and the condenser speakers.

Although I have shown and described certain specific constructions, Iwant it understood that my invention is not limited to the details setforth. It is to be expected that changes and modifications will occur tothose scope of the invention as defined in the appended claims.

I claim as my invention:

1. The combination of a motion picture screen and a plurality ofelectrostatic loudspeakers arranged around-said screen, saidloudspeakers being flat structures mounted substantially parallel withsaid screen and having different natural frequencies to accentuate theentire musical scale within practical limits.

2. In'talking movie apparatus, a curtain having a motion icture screen,and an electrostatic loudspea er mounted on said curtain adjacent saidscreen, the vibratory electrode of said loudspeaker being open to thefree air at the front of said curtain.

3. In talking movie apparatus, a curtain having a motion picture screen,and a plurality of electrostatic loudspeakers mounted on said curtainaround said screen, said loudspeakers being arranged to throw the soundin a forward direction.

4. In talking movie apparatus, a curtain having a motion picture screen,and a plurality of electrostatic loudspeakers mounted on said curtainadj acentsaid screen, said loudspeakers having different naturalfrequencies to accentuate the entire musical scale within `practicallimits. i 5. ln talking movie apparatus, a curtain having a motionpicture screen, and an electrostatic loudspeaker mounted on said curtainadjacent the screen, said loudspeaker having a. diaphragm arrangedbehind the curtain which is perforated in front of the diaphragm topermit the projection of sound in a forward direction.

6. In talking movie apparatus, the combination of a sheet-like supporthaving a p surface constructed to operate as a motion picture screen.)and an electrostatic loudspeaker mounted on said support near thescreen, said louds 'ker comprlsing a condenser plate o substantiallyrectangular form provided with ribs `near the edge, sald plate extendingbeyond said ribs to form outer lflan l, a vibratory electrode mounted onsai plate in tensioned condition and held spaced from said plate by saidribs, fastening Vmeans for said Vvibratory electrode engaging saidflanges, a frame on one side 'of said support for holding said condenserunit to the other side of said support, and fastening devices engagingsaid frame and unit and through saidsheetis rigidly mounted on saidsupport.

7 In talking movie apparatus, the combination of a sheet-like supporthaving a surface constructed to operate as a motion picture screen, a atelectrostatic loudspeaker unit mounted on the rear side of said supportnear the screen, said support in front of said loudspeaker unit beingpervious to the paswhereby said loudspeaker unit un1t.

8. In talking movie apparatus, the combination of a sheet-like supporthaving a surface constructed to operate as a motlon picture screen, aiiat electrostatic loudspeaker unit mounted on one side of said supportnear the screen., a member on the other side of said support, andfastening means passing through said member and unit for securing thelatter to said support. v

9. The combination of a theater curtain, a motion picture screen on saidcurtain, and electrostatic loudspeaker mechanism mounted on said curtainnear said screen and arranged to throw the sound forward, said mechanismbeing surrounded by the curtain which thereby acts as an acoustic baleto l direct sound waves forward.

10. An upright fiat support having a portion constructed to operate as amotion picture screen, and a plurality of electrostatic loudspeakerunits mounted on said support t adjacent said screen and arranged tothrow the sound forward, said loudspeaker units having different naturalfrequencies to accentuate the entire musical scale withinpracticallimits.

